The Concept of HIGH + FAST Load in Tendinopathy

This week the blog focuses on the concept of high + fast load in tendinopathy. We know that stretch shorten cycle load relates to tendon injury but why? And how does this influence our management and rehab planning?

If your patient asked what causes tendinopathy what would you reply? Certainly, load would be part of the explanation. Maybe less so if they had a drug induced tendinopathy – endogenous drugs as in autoimmune issues or drugs they have taken (e.g. fluroquinolones).

Load is a big contributor to most tendinopathies. It is surprising however how little we know about a) the loading that causes tendon injury; and b) the mechanisms underlying load resulting in pathology and pain. If we take the first question for example, van der Worp et al 2011 found in their systematic review that load parameters such as frequency and hours/week were inconsistently linked to patellar tendinopathy. This may be partly explained by change in load..to quote Tim Gabbet et al. ‘High training workloads alone do not cause sports injuries: how you get there is the real issue’.

But my focus with this blog is not to deal with these huge questions – I’ll leave that to the smart people. What I do want to do is briefly discuss the concept of high+fast load.

Something that does seem to be relatively clear is that people who do stretch shorten cycle (SSC) type activity get tendinopathy. Stretch shorten cycle is the fast stretch of the muscle tendon unit followed by the fast recoil (see figure below). A lot of it happens in the tendon (stretch-recoil) with the muscle remaining near isometric which makes metabolic sense (i.e the muscle. operates optimally and efficiently). Read this classic modelling work from Hof et al. 2002 for an explanation.

In the lower limb, SSC occurs during impact loading e.g. walking, running, jumping, hopping, cutting (throwing in the upper limb involves SSC). You can think about the leg/tendon as a spring. You impact load, the spring absorbs energy and compresses, and then the spring recoils and releases some of the stored energy.

The key element of stretch shorten cycle is speed. This is demonstrated well in this work by Kubo et al. 2000. They asked participants to perform slow (0.3 Hz so about 1 in 3 seconds) concentric-eccentric phase calf raises versus fast (1Hz so 1 in 1 second) calf raises. The tendon elongated more (7.1mm vs 10.8mm) and the fascicles less (6mm vs 10mm) in the fast calf raise, meaning more of the work was done by the tendon (42.5% vs 20.1%).

So, fast SSC movements are good for efficient human movement, but why are they related to tendon injury? The superficial explanation is that they are higher load for the tendon. If you take the tasks in the Kubo study discussed above, the ground reaction force was close to 1 body weight for the slow calf raise (did not rise appreciably) versus 1.5 body weights in the fast calf raise, so in this case tendon loads probably are higher for the tendon.

But, this is not always the case. Take this patellar tendon example in the slide below….(note: horizontal landing refers to the takeoff of a horizontal run and then jump as in a spike jump)

Note that the tendon load is high in both tasks (about 5 bodyweights). The key difference is much higher tendon load rate in the SSC jump vs squat task (38 vs 1-2 bodyweights/sec). Also note the time under tension is much lower in the SSC jump vs squat task (about a quarter of a second vs 5 seconds). This indicates that speed of tendon loading is probably a key factor for development of injury.

But it takes both high+fast load. High load is needed to produce tendon strain, and then the rate of that strain is probably important for injury. Probably less likely to be a problem if you have one without the other. Take the clinical example of ITB pain. Often running faster is less painful than running slower. This is probably because the magnitude of ITB load (and therefore strain) is reduced with faster running (probably due to less hip frontal plan excursion and/or greater hip sagittal plane contribution to faster running). So even if the increase in speed causes faster ITB strain the overall strain may be less, so this has less impact.

Take another example. Often Achilles patients have pain with submaximal hop, but not a maximal hop. Although a maximal hop may involve greater joint excursion, it is slower and the knee contributes much more. Again, the balance of high+fast load in the Achilles is a consideration.

The key clinical message from this is that we need to understand how high+fast loads interact for the activities that our patients need to do/return to. For example, what is the more potent combination of high+fast load for a proximal hamstring tendinopathy patient, walking fast or slow running? The modelling data from Sasaki et al. 2006 suggests walking fast results in greater passive and active work than running slow (Look at W120 and R120 which is walking and running at 120% of preferred walking speed, shown during the stance phase). So, what would you progress to first in your return to function/sport rehab?

Here are some summary message:

1. Tendons store and release energy

2. This probably occurs a little with most activities, but especially when speed is involed

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